1. Field of the Invention
This invention relates to conversion lenses, and more particularly to conversion lenses releasably attached on the image side of a master lens system to change the focal length of the entire system. Still more particularly, it relates to conversion lenses of reduced size while still permitting the imaging performance to be improved, suited to be used in photographic cameras or video cameras.
2. Description of the Related Art
In the past, a wide variety of conversion lenses releasably attached on the object or image side of the master lens system to change the focal length of the entire system while maintaining constant the position of the focal plane of the entire system have been proposed. Of these, the so-called front conversion lenses which are attached to the object side of the master lens system tend to have an increased diameter. Therefore, it was difficult to achieve a reduction of the size of the entire system. Meanwhile, the so-called rear conversion lenses which are attached to the image side of the master lens system are advantageous in reducing the lens diameter as compared with the above-described front conversion lenses, but their aberration correction is difficult to achieve. Therefore, the number of lens elements had to be increased, and the total length of the lens tended to increase.
Japanese Laid-Open Patent Application No. Sho 54-97423 proposes a rear conversion method in which the conversion lens is inserted into a space after the master lens system has been axially moved forward to change the focal length of the entire system. But, because the number of lens elements is as many as six, the total length of the lens tends to increase.
Japanese Laid-Open Patent Application No. Sho 58-195817 discloses a conversion lens of relative small size in which the rear conversion method is also employed and, while a shortening of the back focal distance of the entire system is achieved, the focal length of the entire system is changed. But, the change of the focal length is 1.25 times. So the magnification change rate was not always sufficient.
In general, to achieve a high magnification change rate while maintaining a reduction in size of the entire system, the refractive power of the conversion lens may be strengthened. This method had drawbacks that too much aberration was produced, and particularly spherical aberration and Petzval sum were increased so that correction of curvature of field became difficult to perform.
Japanese Laid-Open Patent Application No. Sho 56-94318 filed by the assignee of the present invention discloses a rear conversion lens comprising two lens elements of positive and negative powers. The photographs taken with this lens attached to the master lens when enlarged in a high rate, sometimes impressed the viewer with a loss of the image quality.